In modern mobile communication networks, the current position determination technologies utilize one or more of four fundamental methods of determining a subscriber's location before it defaults to the location of the serving sector. There are two ranging technologies (e.g., Assisted GPS or AGPS and Advanced Forward Link Trilateration or AFLT). These can be used independently or together and a default solution can be identified by a point associated with the coverage footprint of the serving sector in the mobile network. Both ranging technologies use the mobile device assets to report the relative times of arrival (“TOA”) or time difference of arrival (“TDOA”) of signals from ranging elements, either GPS satellites or terrestrial base stations, to a network server called a Position Determining Entity (“PDE”). The PDE knows the location of the ranging elements and combines that knowledge with the TOA or TDOA information to generate distances to the mobile device that is the target of the position determination. From the distances, position can be determined. Determining position based upon distance measurements to various known locations of signal source is called Trilateration.
Traditionally, a one-hundred percent (100%) GPS solution provides the most accurate position solutions. This, however, requires sufficient number of satellites be receivable or “visible” for position determination. When the mobile device receives signals from an insufficient number of satellites, a hybrid solution is used, utilizing ranging or distance measurements both from GPS satellites and terrestrial base stations. When three or more base stations are visible, a position fix can be determined even if no satellites are visible. This technique is called Advanced Forward Link Trilateration or AFLT. Lastly, in a scenario in which there are insufficient combinations of satellites and base stations to generate a position estimate, the system may give up calculating an accurate position and may retreat back to the default location of the serving sector as its best guess at a position estimate.
In-building subscribers make up a significant percentage of voice (˜70%) and data (˜85%) call volumes. Service providers, such as Verizon Wireless™, have many programs in place to bring quality voice and data to in-building subscribers. In-building subscribers, by definition, do not have an unobstructed view of the sky and therefore position determination performance will be compromised due to lack of GPS signals. Furthermore, many in-building solutions involving repeaters and Distributed Antenna Systems (“DAS”) can further degrade position determination system performance through the creation of ambiguities in the distance measurements.
Repeaters or bi-directional amplifiers are frequently used to bring RF energy into buildings where subscribers lack coverage ubiquity or capacity sufficiency. Even in a simple configuration with a single donor and single coverage antenna, the process of augmenting voice and data service through these devices delays the RF energy captured from the network as it passes through the repeaters. This delay has been shown to have adverse impacts on subscriber position determination in a variety of RF circumstances known to exist in the macro RF network.
Repeaters or other base station network elements can also feed a DAS. The DAS may have both active and passive components. The circuitous pathways of signal travel, delays caused due to signal speed through the fiber being less than the speed through the air, and delays through active components further decouple signal arrival measurements and distance estimates to reference locations. Further, network optimization protocols often reduce the power that the macro network broadcasts into the DAS-covered venue. This may result in weaker signals of surrounding base stations available for AFLT distance determination between subscriber and ranging reference base stations.
In large venues, where little (if any) GPS signals are available and dedicated base station coverage and capacity is provided through a sophisticated DAS infrastructure, position determination algorithms are further challenged. In such an environment, the base station energy from a single sector is simulcast throughout a predetermined coverage footprint. Simulcasting often refers to a process by which energy is distributed to multiple areas of interest, which may or may not overlap, through different pathways and devices. The process of simulcasting decouples the relationship between the TOA measurements and the references mobile device's location, introducing large errors/uncertainties into the position solution.
Therefore, there is a need for a system and a method that improves the location determination for mobile device users who do not have unobstructed view of the sky. In particular, there is a need for a system and a method that negates or reduces the impacts of DAS simulcasting requirements, noise reduction or artificially introduced delay on the AFLT position determination performance. Similarly, there is a need for a system and a method that negates or reduces the impact of delays that is caused by the repeaters in a non-DAS environment.